Ford ka occasional stiff steering

Anyone any ideas whats causing this problem? Occasionally....about once every 2 weeks the steering creates initially a grating/grinding sensation then stiffens up with no further grinding
sensation. This problem appears to be initiated when a right hand turn was taken followed by a left hand turn to straighten the vehicle up. This cannot be repeated for the ford dealer to make an assessment of the problem because it cannot be repeated at will. My daughter who is the driver cannot be certain if it clears after stopping the engine or if it clears by moving the steering wheel to extremes in turning a corner. The PAS hyrdraulic reservoir is at the required level and looks to be clear. The KA is Sept 2000 X reg.
Any suggestions...and hopefully it isnt going to be the steering rack!!!

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